The invention relates to a contamination monitor for measuring a degree of contamination in an evacuatable charged particle beam system, comprising a carrier with an aperture for transmitting a charged particle beam in a beam propagation direction.
A contamination monitor of this kind is known from W. R. Bottoms "Contamination in the SEM", Scanning Electron Microscopy/1973, Proceedings of the Sixth Annual Scanning Electron Microscope Symposium, pp. 181-188.
The cited article describes a measuring method where the contamination monitor in a scanning electron microscope (SEM) comprises a carbon film with an aperture. The carbon film with the aperture is scanned by an electron beam and an enlarged image of the aperture is displayed on a television monitor. During irradiation of the carbon film by the electron beam, the aperture is clogged due to beam-induced polymerization of hydrocarbon molecules. These hydrocarbon molecules are present in an evacuatable charged particle beam system due to oil from vacuum pumps, vacuum sealing materials, materials present on the walls of the system, or specimens studied in the beam particle system. A rate of contamination can be measured by determining the decrease of an aperture diameter. A chemical composition of the contamination can be determined by way of Auger measurements. However, a quantitive measure of the contamination is not obtained in this manner.